In the oil and gas industry, acoustic tools are used to provide measurements of the attributes (such as slowness, time, coherence, coherent energy, attenuation and instantaneous frequency,) of various types of sonic waves propagated from transmitter to receiver. These attributes are analyzed to help estimate, among other things, the permeability and the mobility of the fluid content of the formation. These estimates are the basis for critical assessments concerning the rate of flow of a hydrocarbon (gas or oil) out of a producing borehole. Collecting, recording, and analyzing the seismic wave attributes to obtain information about the formation and the hydrocarbon contained within it on a delayed or real time basis is known as well logging.
Evaluation of physical properties such as pressure, temperature and wellbore trajectory in three-dimensional space and other borehole characteristics while extending a wellbore is known as measurements-while-drilling (MWD) and is standard practice in many drilling operations. MWD tools that measure formation parameters such as resistivity, porosity, sonic velocity, gamma ray, etc. of a formation are known as logging while drilling (LWD) tools. Information that can help the driller make important and timely decisions about the drilling program are indicators of fracture and permeable zones in a formation on a real time basis.
For the above and other reasons, the oil industry has developed acoustic well logging techniques that involve placing an acoustic tool within a well bore to make measurements indicative of formation attributes such as compressional slowness (DTc), shear slowness (DTs) and Stoneley slowness (DTst). Sonic logs can be used as direct indications of subsurface properties and, in combination with other logs and knowledge of subsurface properties, can be used to determine subsurface parameters, such as those related to borehole structural stability, that can not be measured directly. Early efforts in this connection were reported by Rosenbaum in “Synthetic Microseismograms: Logging in Porous Formations”, Geophysics, Vol. 39, No. 1, (February 1974) the disclosure of which is incorporated by reference as though set forth at length.
Acoustic logging tools typically include a transmitter and an array of axially spaced acoustic detectors or receivers. These tools are operable to detect, as examples, formation compressional waves (P), formation shear waves (S) and Stoneley or tube waves (St). These measurements can be performed following drilling or during intermediate drill string trips by wireline logging operations. In wireline logging, sonic monopole tools can be used to excite and detect compression waves (P) and Stoneley waves (St) in all formations and shear waves (S) in fast formations. In addition to wireline logging, techniques have been developed where piezoelectric transmitters and hydrophone receivers are imbedded within the walls of drill string segments so that sonic LWD operations can be performed.
Early LWD and sonic data processing techniques developed by the Schlumberger Technology Corporation such as a slowness-time-coherence (STC) method is disclosed in U.S. Pat. No. 4,594,691 to Kimball et al. entitled “Sonic Well Logging” as well as in Kimball et al. “Semblance Processing of Borehole Acoustic Array Data,” Geophysics, Vol. 49, No. 3 (March 1984). This method is most useful for non-dispersive waveforms (e.g. monopole compressional and shear head waves). For processing dispersive waveforms a dispersive slowness-time-coherence (DSTC) method is preferred. This process is disclosed in U.S. Pat. No. 5,278,805 to Kimball entitled “Sonic Well Logging Methods and Apparatus Utilizing Dispersive Wave Processing.” The disclosures of these patents, of common assignment with the subject application, as well as the noted Geophysics publication authored by an employee of Schlumberger are hereby also incorporated by reference.
Sonic wireline tools, such as a Dipole Shear Sonic Imager (DSI—trademark of Schlumberger) and Schlumberger's Sonic Scanner generally have a multi-pole source. A multi-pole source may include monopole, dipole and quadrupole modes of excitation. The monopole mode of excitation is used traditionally to generate compressional and shear head waves such that formation compressional and shear slowness logs can be obtained by processing the head wave components. The head wave components are non-dispersive and are generally processed by slowness-time-coherence (STC) methods as discussed in the Schlumberger Kimball et al. '691 patent and Vol. 49 Geophysics article noted above.
The slowness-time-coherence (STC) method is employed to process the LWD sonic waveform signals for coherent arrivals, including the formation compressional, shear and borehole Stoneley waves. This method systematically computes the coherence(C) of the signals in time windows which start at a given time (T) and have a given window move-out slowness (S) across the array. The 2D plane with slowness on the y-axis and time on the x-axis is called the slowness-time-plane (STP). All the coherent arrivals in the waveform will show up in the STP as prominent coherent peaks. The compressional, shear and Stoneley slowness (DTc, DTs, and DTst) will be derived from the attributes of these coherent peaks.
The response of the Stoneley wave to open micro-fractures and permeable pore zones is essentially the same. In the past, Schlumberger U.S. Pat. No. 4,964,101 has described how to use wireline-measured Stoneley wave slowness and attenuation to detect these zones and compute the fluid mobility over these zones. The disclosure of this Schlumberger '101 patent is incorporated by reference as though set forth at length. In this prior '101 patent disclosure, the logging tool is considered acoustically transparent, i.e. a model of fluid-filled borehole through a permeable formation is employed. In logging while drilling operations using sonic transmitters and sensors carried by a drill string, the presence of the rigid drill collar changes the behavior of the Stoneley wave significantly. For example, the attributes of the wave will generally be more sensitive to fluid mobility than in the case of a fluid-filled borehole without the drill collar.
In the prior Schlumberger U.S. Pat. No. 4,964,101 patent a key component was the specific way to address mud cake effects on the Stoneley dispersion curve. In an LWD environment, due to the drill collar agitation and a short time lag between a borehole drilling operation and LWD measurements, it is generally expected that mud cake on the borehole wall will not have been formed and thus will not be a significant factor when LWD measurements are taken. Instead with a LWD drill string operation the effect of a relatively large diameter and heavy drill collar in the borehole is a significant factor for consideration and accommodation.
Although measuring Stoneley wave slowness and attenuation to detect micro-fractures and permeable pore zones in the past has been used with wireline tools, it would be desirable to make LWD measurements using a drill string that includes a thick-walled, drill collar.